For certain communications protocols, a transceiver only transmits or receives at any given time, and does not transmit and receive simultaneously. This allows a chip incorporating the transceiver to use two differential pins for both receiving a received signal and providing a signal to be transmitted. This reduces the pin count compared to chips that have separate pairs of differential pins for the received signal and the signal to be transmitted. The pins are therefore used for both input and output.
It is necessary in the transceiver to switch from a receive mode to a transmit mode and vice versa. In the receive mode, the transceiver processes the received signal using a receiver. A transmitter in the transceiver does not produce a signal to be transmitted. In the transmit mode, the transmitter produces a signal to be transmitted, and the receiver does not process the received signal.
The transceiver is typically implemented with switches in the signal path of the transmitter and the receiver. The switches connect the input/output pins to either the transmitter or the receiver, depending on the mode (transmit or receive) of the transceiver. The switches are therefore connected in series between the pins and the input nodes of the transmitter and the receiver.
As the switches are connected in series with the transmitter and the receiver, the switches introduce extra resistances that degrade the noise performance of the transceiver. For example, the extra resistances degrade noise performance of the receiver, and/or increase losses for the transmitter.
It is an object of embodiments of the invention at least to mitigate at least some of the problems associated with the prior art.